NdFeB (neodymium-iron-boron) system sintered magnets were discovered in 1982 by Sagawa and other researchers. NdFeB system sintered magnets exhibit characteristics far better than those of conventional permanent magnets, and can be advantageously manufactured from raw materials such as Nd (a kind of rare earth element), iron, and boron, which are relatively abundant and inexpensive. Hence, NdFeB system sintered magnets are used in a variety of products, such as voice coil motors used in hard disks and other apparatus, driving motors for hybrid or electric cars, battery-assisted bicycle motors, industrial motors, high-grade speakers, headphones, and permanent magnetic resonance imaging systems.
Three methods have been previously known to be available for producing NdFeB system sintered magnets: (1) sintering method; (2) casting method including the process steps of casting, hot working and aging; and (3) die-upsetting method including the step of die upsetting a rapidly cooled alloy. Among these methods, the sintering method is superior to the other two methods in terms of the magnetic properties of the products and the productivity, and has already been established on the industrial level. With the sintering method, dense and uniform microstructures necessary for good permanent magnets can be obtained.
Patent Document 1 discloses a method for producing a NdFeB system sintered magnet by a sintering method. A brief description of this method is as follows: Initially, a NdFeB alloy is created by melting and casting. This alloy is pulverized into fine powder, and the fine powder is put into a mold. A magnetic field is applied to this alloy powder, while pressure is applied to the powder with a pressing machine. In this process, both the creation of a compact and the magnetic orientation of the compact are simultaneously performed. Subsequently, the compact is removed from the mold and heated for sintering to obtain a NdFeB system sintered magnet.
Fine powder of a NdFeB system alloy is easily oxidized and may ignite by reacting with oxygen in air. Therefore, the previously described process should preferably be performed entirely in an airtight container in which no oxygen is present or inert gas is filled. However, this is impractical because creating the compact requires a large-sized pressing machine capable of applying a high pressure of tens or hundreds MPa to the alloy powder. Such a pressing machine is difficult to be set within an airtight container.
Patent Document 2 discloses a method for producing a sintered magnet without using a pressing machine (i.e. without creating a compact). This method includes the three processes of filling, orienting and sintering, which are performed in this order to create a sintered magnet. A brief description of this method is as follows: In the filling process, an alloy powder is supplied into a mold, after which the density of the alloy powder in the mold is increased by a pushing, tapping or similar operation to a level of approximately 3.0-4.2 g/cm3, which is higher than a natural filling density and lower than a density of the press compact. In the orienting process, a magnetic field is applied to the alloy powder, without applying any pressure, to orient the particles of the alloy powder in the mold in one direction. In the sintering process, the alloy powder which has been aligned in one direction in the orienting process is heated, together with the mold, to be sintered. In this method, since no pressure is applied to the alloy powder in the magnetic orienting process and the density of the alloy powder is lower than that of the compact in the press-molding process, the friction among the particles of the alloy powder is reduced. Accordingly, in the orienting process, the powder particles can be aligned with high degree of orientation. As a result, a NdFeB system sintered magnet with even higher magnetic properties is obtained.
Patent Document 2 also discloses a system for producing a sintered magnet using an airtight container in which no oxygen is present or inert gas is filled, and in which a filling unit, an orienting unit and a sintering unit are provided together with a conveyer for moving the filling container from the filling unit to the orienting unit and then from the orienting unit to the sintering unit. In this system, the alloy powder is handled under oxygen-free or inert-gas atmosphere throughout the entire process, so that the oxidization of the powder and the deterioration of magnetic properties due to the oxidization will not occur. Such a method in which no press compact is created, and in which the alloy powder is held in a mold until it is sintered into a magnet, will be hereinafter referred to as the “press-less process” or “PLP” method.